Xmake's biggest advantage: it's a single binary (vs Meson requiring Python)
Meson's biggest advantage (aside from popularity): it's a declarative DSL rather (vs including a full-blown programming language like Lua in Xmake)
(The second point is subjective, I know some think that including a full programming language is a strength but to my eyes it's a downside that largely outweighs the Python dependency.)